Parasite control programs have traditionally involved regular rotation of anthelmintics (dewormers). This has been useful for controlling large strongyle infections, which are now considered rare largely due to these regular deworming programs. Cyathostomes (small strongyles), tapeworm (Anoplocephala perfoliata) and ascarids (Parascaris equorum) are currently recognised as the major parasites affecting horses. These worms can have high levels of anthelmintic resistance likely associated with the widespread routine treatment programs.

What is anthelmintic resistance?

Anthelmintic resistance is the ability of worms to survive treatments with dewormers that would normally be effective in killing the same species of worms at the same stage of development. Resistance occurs when worms that survive anthelmintic treatment reproduce and pass on their genes conveying natural resistance. This creates a population with increased resistance genes due to selection pressure from regular deworming. Repeated anthelmintic treatments promotes the survival of parasites with increasing resistance as they don’t have to compete with as many other worms and can readily reproduce. It is this reason that the concept of refugia becomes an important component of parasite management.

What is refugia?

Refugia refers to the portion of a worm population that are not exposed to anthelmintic treatment. This can include worms within an untreated horse, encysted cyathostomes, and those parasite stages on the pasture. These populations remain susceptible to anthelmintics and act to dilute the resistant worms. Therefore, keeping anthelmintic treatments to a minimum when refugia is low (cold weather and drought) is important to reduce selection for resistant worms. Another way to reduce development of resistance is to leave a few horses untreated. Some horses can naturally shed a higher number of eggs onto the pasture compared to others and are considered high shedders, with 20-30% of adult horses shedding approximately 80% of the eggs on pasture.

How do I determine which horses to treat or not to treat?

Faecal Egg Counts (FEC) are a useful tool to monitor the effectiveness of parasite control programs and to monitor resistance patterns to treatment. A FEC involves collecting a sample of manure and diluting it down to examine its contents microscopically looking specifically for parasite eggs. This can easily be performed by your veterinarian. Currently, we recommend that horses with FEC >200 eggs per gram receive anthelmintic treatment as they are considered moderate contaminators, whilst >500 eggs per gram are high contaminators. Strongyle eggs can be attributed to either large strongyles, small strongyles (cyathostomes) or threadworm, however, 99% of all strongyle eggs found on FEC can be attributed to cyathostomes.

To determine the exact worms present a culture would have to be performed where the eggs develop to larvae in a controlled environment allowing accurate identification.

What types of parasites affect horses?

Cyathostomins (small strongyles or small red worms) are currently considered the most prevalent and pathogenic parasites. These parasites can rapidly develop on pasture from egg to infective larvae in as little as 3 days in warm, moist environments. The larvae can also arrest development within intestinal mucosa when the environmental conditions outside are not ideal. These larvae can remain in this encysted state for up to 2 years and evade most anthelmintic treatments. The significance of encysted cyathostomins is the resulting clinical disease of ‘larval cyathostominosis’ associated with serious mucosal damage that occurs at emergence during late Winter and early Spring. This condition can have a mortality rate as high as 50% and commonly presents as chronic diarrhoea, colic , anorexia and weight loss. Treatment of cyathostomins is important in horses of all ages as they remain susceptible to infection throughout their lives with clinical disease occurring at any age and in any season.

Tapeworms

Tapeworm infections are difficult to diagnose using faecal egg counts as the eggs are intermittently shed from horses. Thus, an infected horse may have a negative FEC. Another diagnostic option is to treat the horse for tapeworm and take a faecal sample 24 hours later as this method will increase the chance of seeing tapeworm eggs on FEC. A variety of laboratory tests are available to determine possible exposure or infection with tapeworms, however, these are not routinely performed. Tapeworms are considered widespread, have the potential to cause disease and are difficult to diagnose so treatment is recommended. One treatment during winter with praziquantel is generally sufficient in controlling tapeworm.

Ascarids

Generally, this parasite is of greater importance in foals as it can cause poor growth and overall poor health. Ascarids survive in the environment for many years and are often widespread within breeding operations. Typical signs of infection include coughing and nasal discharge as the larvae migrate into the airways, and possibly colic due to small intestinal impactions. High levels of resistance have been documented in pinworms so consultation with your veterinarian is recommended to determine the correct anthelmintic treatment.

Pinworms

Pinworms are typically seen in young horses, however, cases in adults are becoming more common with not every horse within a group will be affected. Clinical signs usually involve tail rubbing and hindquarter/perineal skin excoriations, although some horses may not show any specific clinical signs. The pinworm eggs are spread when the horse rubs against objects and transmission can occur in stalls and contact with infected objects. Diagnosis can be made by a veterinarian through microscopic identification of eggs around the anus using a sticky tape test or skin scraping. Like ascarids, resistance has been noted in pinworm populations so consultation with your veterinarian is recommended. In addition to treating your horse, it is important to clean the perianal region to reduce the itch associated with the eggs and give the horse some comfort. Any cleaning material needs to be disposed of or disinfected post treatment.

What should I consider when making a parasite control program for my horses?

Ultimately, any parasite control program should aim to reduce risk of parasitic disease, reduce potential egg shedding and reduce development of anthelmintic resistance. The following points are important to consider when tailoring a program for your own individual needs:

1. DO remove faeces from pasture. As strongyles develop within manure prompt removal of faeces will reduce risk of ingestion by a horse. Twice weekly manure removal can be effective at controlling worm infection on pasture.
2. DO compost manure and bedding to expose larvae to higher temperatures and kill the parasite.
3. DO consider grazing infected pastures with sheep and cows to help reduce strongyle worm burden on pasture as they are equine specific.
4. DO perform regular FEC prior to treatments to target horses with higher shedding potential.
5. DO focus anthelmintic treatments at the time of year that are optimal for larval development. This would involve treatment with a larvicidal drug (moxidectin or double dose fenbendazole) in Autumn or Spring.
6. DO provide twice yearly anthelmintic treatments for ALL horses. This is important for the management of large strongyles, tapeworms and bots and is generally enough for most horses.
7. DO NOT UNDERDOSE- get an accurate weight using scales or a weigh tape (contact your local equine hospital to discuss weighing your horse).
8. DO NOT treat during colder winter months and hot summer months to reduce development of resistance.
9. DO NOT overstock paddocks as high numbers of horses will result in greater parasite exposure.
10. If you have any concerns, please speak to your equine veterinarian.

Written by Dr Shaunna McTernan